The invention is directed to a dispersion flattened single mode optical waveguide fiber designed for soliton transmission. Further, the invention relates to a dispersion flattened single mode optical waveguide fiber designed to propagate wavelength division multiplexed soliton signals.
It is well known in the art that, because of the non-linearity of the refractive index of SiO.sub.2 based waveguides, soliton propagation in such waveguides is possible. A pulse having a prescribed intensity and shape, propagating in a waveguide fiber, will undergo a non-linear self phase modulation (SPM) which produces a pulse spreading, longer wavelengths shifted forward relative to shorter wavelengths. If the waveguide fiber has a total dispersion, D (in units of ps/nm-km of proper magnitude and positive sign (shorter wavelengths travel at higher speed relative to longer wavelengths)), this total dispersion will serve to cancel with the SPM pulse spreading. An alternative statement of the soliton effect is, the non-linear SPM of pulses, which is the dependence of group velocity change on pulse intensity and shape, effectively cancels the frequency dependence of group velocity change.
The transmission of solitons in optical waveguide fiber has been reported by several workers, e.g., Mollenhaur et al., "Demonstration of error-free soliton transmission over more than 15000 km at 5 Gbits/s, single channel, and over more than 11000 Gbits/s in two channel WDM," Electron. Letters 28(8), 792-794(1992). The two channels were separated by 0.4 nm. It is likely that a large dispersion slope of the waveguide forced this small channel spacing and limited the wavelength division multiplexing to only two channels.
With a dispersion flattened single mode waveguide the number of wavelengths multiplexed and the channel spacing could be increased by about a factor of five, providing much larger capacity, less cross channel interference, and wider latitude on tolerances of the multiplexed signals.
Thus, the combination of dispersion flattened waveguide fiber with soliton information transmission provides a powerful tool for increasing waveguide fiber capacity and increasing spacing between regenerators.
Further, a waveguide fiber having a flat slope, greatly simplifies the process of maintaining the intensity required for soliton propagation over several channels. The signal intensity or power required for soliton creation is directly proportional to group velocity dispersion. A dispersion flattened waveguide provides uniform group velocity dispersion over an extended wavelength range. Hence, threshold power for soliton creation is reduced and power from channel to channel is substantially equal. In addition, the tolerance on center frequency of the soliton signal pulses is broadened.
Additional benefits result from the combination of dispersion flattened waveguides and soliton transmission in systems which include optical amplifiers. Very long unregenerated systems, for example the system described in the Mollenhaur et al., Electron Letters, publication cited above, require optical amplifiers. Present technology favors the short length (lumped) erbium doped fiber amplifier (EDFA) technology. However, workers continue to direct their research toward wider wavelength ranges and flatter amplifier gain curves with erbium optical amplifiers as well as optical amplifiers having a variety of alternative compositions.
In a typical waveguide fiber, carrying solitons of different center wavelengths, and thus different speeds, solitons can pass through one another. In a lossless waveguide, for example a waveguide incorporating a distributed optical amplifier, the solitons which pass through one another are essentially unchanged. There is no change in central frequency, shape or intensity of the soliton pulses. However the dispersion flattened waveguide fiber still provides a benefit. As noted above, soliton power depends directly upon dispersion. Also, the amplifier gain curve as a function of wavelength is not flat. In a non-dispersion flattened waveguide fiber design, the solitons of different center wavelength have different threshold power, because dispersion varies with wavelength and threshold power depends upon dispersion. The difference in soliton power may be magnified by the dependence of amplifier gain on wavelength. Thus, starting the solitons of different central wavelength at about the same power level, as can be done in a dispersion flattened waveguide fiber, will tend to minimize soliton power differences due to variation of amplifier gain with wavelength.
The advantage of a dispersion flattened waveguide fiber is greater when short length (lumped), i.e., non-distributed optical amplifiers are used. Mollenauer et al.,"Wavelength Division Multiplexing with Solitons in Ultra-Long Distance Transmission Using Lumped Amplifiers", Journal of Lightwave Tech., V.9, #3, March 1991, have shown that the solitons do not interact when passing through one another (collide), in systems using short length (lumped) optical amplifiers, provided the length of waveguide over which the solitons collide is long relative to the optical amplifier spacing.
The benefit derived from using a dispersion flattened waveguide in the soliton pulse system is clear. Because dispersion slope is small for this waveguide fiber, the difference in speed between solitons of different center wavelength is smaller than that for, say, a standard dispersion shifted waveguide fiber. The result is that the waveguide length over which a soliton collision occurs is increased, and thus a collision occurs over several amplifier spacing lengths, thereby minimizing the effect of the lumped amplifiers on soliton center wavelength or relative position along the waveguide.
In summary, the use of the combination of soliton signal pulses together with dispersion flattened waveguide fiber provides:
an increased range of wavelengths for wavelength division multiplexing; PA1 a minimization of soliton pulse power variation, in systems using distributed amplifiers due to variation of amplifier gain with wavelength; PA1 a minimization of temporal or center wavelength shifts of solitons colliding in systems using lumped optical amplifiers; PA1 a larger tolerance on soliton center wavelength; PA1 more multiplexed signals having a channel spacing sufficient to reduce channel cross talk and relax wavelength division multiplexing constraints; PA1 a reduced required power level and power level control on wavelength division multiplexed soliton signal pulses; and, PA1 a reduced dispersive effect due to different polarization modes by reducing intensity level dependent waveguide bi-refringence.
The inventive combination of soliton signal pulses and dispersion flattened waveguide fiber may be expected to play a major role in the many planned installations of high performance, long distance waveguide fiber telecommunications systems. This will be especially true for those high performance systems incorporating lumped optical amplifiers.